The present invention relates to an optical scanning device comprising an optical objective lens. More specifically, but not exclusively, the invention relates to an optical scanning device capable of writing and reading data from two different types of optical record carriers, such as compact disc (CD) and digital versatile disc (DVD), with laser radiation of a different wavelength, respectively, using a single optical objective lens system.
It is desirable for an optical scanning device to be capable of recording and reproducing optical record carriers of different formats. CDs are available, inter alia, as CD-As (CD-audio), CD-ROMs (CD-read only memory) and CD-Rs (CD-recordable). CDs are designed to be scanned with a wavelength of about 780 nm and a numerical aperture (NA) of 0.45, DVDs are designed to be scanned at a wavelength in the region of 660 nm. For reading DVDs an NA of 0.6 is generally used, whereas for writing DVDs an NA of 0.65 is generally required.
DVDs and CDs differ in the thickness of their transparent substrates, providing different information layer depths. The information layer depth for DVD is about 0.6 mm, whereas the depth for CD is about 1.2 mm. Thus if CDs are read with an optical scanning device with an objective lens optimised for DVD, a large amount of spherical aberration results at the information layer. It is possible to compensate for this effect when using a single objective lens system and a laser beam of 660 nm wavelength for reading both CD-ROMs and DVDs by reducing the numerical aperture (NA) for reading CD-ROMs from about 0.45 to 0.38, causing the spherical aberration to be within the limits for a proper reconstruction of the information stored on the CD-ROM. However, if in addition CD-Rs are to be scanned by the optical scanning device, a 780 nm laser beam has to be applied, because the CD-R is designed for writing and reading specifically at that wavelength. For CD-R organic dye is used as a recording film, of which the reflection characteristics change significantly with wavelength. It is difficult to achieve sufficient modulation for reflected radiation of 660 nm to reconstruct the information stored on a CD-R. Using a 780 nm laser beam and an NA of 0.45 for reading CDs with the same objective lens as used for DVD causes a large spherical aberration. Therefore the spherical aberration has to be compensated in some way in order to achieve an optical storage device capable of reading and/or recording CD-R, CD-ROM and DVD using laser radiation of 660 nm and 780 nm with a single optical objective lens.
Systems capable of reading DVD and CD by using laser radiation of different wavelengths with the same objective lens are known in the art. WO 99/57720 describes such a system, which uses a moulded plastic lens having either two refractive aspheric surfaces or one aspherical surface and one refractive spherical surface including a diffractive element. The lens is capable of correcting for spherical aberration caused by the difference in thickness for the two disc formats as well as for chromatic aberration.
EP-A-936604 describes an optical pickup device suitable for reading and writing discs of DVD, CD-R and CD-ROM format with laser radiation of two different wavelengths. For this purpose an optical element with a first diffraction grating pattern in its central region and a second diffraction grating pattern in its peripheral region is used in addition to an objective lens. The central region of the optical element allows transmission of a laser beam of a first wavelength without any change but increases the diameter of a laser radiation of a second wavelength, and uses a stepped grating profile to achieve this. The peripheral region again allows the transmission of laser radiation of the first wavelength without any change and simultaneously shields radiation of the second wavelength from contributing to the spot formation. In this way the NA for use of radiation of the second wavelength is reduced to the desired value. The central part of the element achieves a diffraction efficiency of at least 90% for the zeroth diffractive order of the first wavelength radiation and an efficiency of at least 70% for the first diffractive order of the second wavelength radiation. In an example shown a four step grating profile is used, with each step height in the grating profile increasing in height by the same value h1.
It is an object of the present invention to provide an improved optical scanning device for scanning different optical record carriers using radiation of a first and second wavelength, respectively.
According to one aspect of the invention, there is provided an optical scanning device for scanning optical record carriers with radiation of a first and a second wavelength, the device having a diffractive part, the diffractive part including a pattern of pattern elements having a stepped profile, wherein the total optical paths pertaining to steps of a pattern element are substantially equal to multiples of said first wavelength, such that radiation of said first wavelength is substantially undiffracted by the said diffractive part and such that said device has a first focusing characteristic for said first wavelength, and wherein the steps are arranged such that radiation of the second wavelength is diffracted by said diffractive part and such that said device has a second focusing characteristic, different to said first focusing characteristic, for a selected diffractive order of the radiation of the second wavelength, characterised in that the relative step heights between adjacent steps of said pattern element include a relative step height having an optical path substantially equal to axcex1, wherein a is an integer and a greater than 1 and xcex1 is said first wavelength, whereby an improved efficiency of transmission for said selected diffractive order of the radiation of the second wavelength is provided.
By selecting appropriate relative step heights having optical paths, one or more of which may be multiples of the first wavelength axcex1, radiation of the first wavelength will be substantially undiffracted by the diffractive part, radiation of the second wavelength will simultaneously have a high diffraction efficiency in the selected order, whilst also achieving the desired focusing characteristics. This can be achieved even with a relatively small number of steps in a pattern element, thus improving manufacturing efficiency.
The diffractive part may be operating in reflection or in transmission. When operating in reflection, the diffractive part can be integrated with a mirror used for changing the direction of the optical axis of the radiation in the scanning device. When operating in transmission, the total step heights of steps of a pattern element are preferably substantially equal to multiples of height h1 with             h      1        =                  λ        1                    (                  n          -                      n            0                          )              ,
where n is the refractive index of the material from which the diffractive part is made, and n0 is the refractive index of the adjacent medium, and the relative step heights between adjacent steps of said pattern element include a relative step height having an optical path substantially equal to ah1.
In accordance with another aspect of the present invention there is provided an optical element for use in an optical scanning device for scanning optical record carriers having a first and second information layer depths with radiation of a first and second wavelength and with a first and second numerical aperture respectively, said optical element including a diffractive part, wherein the diffractive part includes a pattern of pattern elements having a stepped profile, wherein the total optical paths pertaining to steps of a pattern element are substantially equal to multiples of said first wavelength, such that radiation of said first wavelength is substantially undiffracted by the said diffractive part and such that said device has a first focusing characteristic for said first wavelength, and wherein the steps are arranged such that radiation of the second wavelength is diffracted by said diffractive part and such that said device has a second focusing characteristic, different to said first focusing characteristic, for a selected diffractive order of the radiation of the second wavelength, characterised in that the relative step heights between adjacent steps of said pattern element include a relative step height having an optical path substantially equal to axcex1, wherein a is an integer and a greater than 1 and xcex1 is said first wavelength, whereby an improved efficiency of transmission for said selected diffractive order of the radiation of the second wavelength is provided.
In accordance with a further aspect of the invention there is provided a method of designing an optical element for use in an optical scanning device for scanning optical record carriers with radiation of a first and second wavelength, said optical element including a diffractive part, wherein in a first part of said method a diffractive part is designed to approximate a generally sawtooth-like structure with a height h2 on the highest side of each sawtooth-like pattern element generating a phase change substantially equal to ixc2x72xcfx80, where i is an integer, for radiation of said second wavelength, the sawtooth-like structure being approximated by an initially selected stepped profile representing said sawtooth-like structure, and wherein in a second part of the method the phase profile of the initially selected stepped profile is approximated by a different selected stepped profile with steps of a total step height approximately equal to h1xe2x80x2, with             h      1      xe2x80x2        =                            λ          1                          n          -          1                    ⁢      k        ,
whereby xcex1 is said first wavelength, n is the refractive index of the material from which the optical element is made, n0 is the refractive index of the adjacent medium and k is an integer taking a plurality of different values within one pattern element.
Such a two-stage design process is useful for generating a diffractive part which has a substantially maximal transmission efficiency for the zeroth diffractive order radiation of the first wavelength, whilst a relatively high efficiency for the selected higher diffractive order radiation of the second wavelength is also ensured.
In accordance with another aspect of the present invention, there is provided an optical scanning device for scanning optical record carriers with radiation of a first and a second wavelength, the device including a diffractive part, the diffractive part including a pattern of pattern elements having a stepped profile, wherein the step heights are selected: such that radiation of said first wavelength passes said diffractive part in zeroth diffractive order substantially unattenuated and is subsequently focused with a first focusing characteristic; and such that radiation of said second wavelength is diffracted by said diffractive part, resulting in a beam of a selected diffractive order, with a second focusing characteristic different from said first characteristic. The radiation passing the diffractive part may be radiation reflecting on the diffractive part or radiation transmitted through the diffractive part.
In accordance with yet another aspect of the present invention, there is provided an optical element for use in an optical scanning device for scanning optical record carriers with radiation beams of a first and second wavelength and with a first and second numerical aperture respectively, said optical element including a diffractive part, the diffractive part including a pattern of pattern elements having a stepped profile, wherein the step heights are selected: such that radiation of said first wavelength passes said optical element in zeroth diffractive order substantially unattenuated and to provide said device with a first focusing characteristic; and such that radiation of said second wavelength is diffracted by said optical element, resulting in a beam of a selected diffractive order, and to provide said device with a second focusing characteristic different from said first characteristic.
By optimising the properties of the stepped profile for the transmission of the first wavelength radiation, the functioning of a device for scanning high density record carriers, like DVDs, at that wavelength, can be improved.